Padding a swift String for printing

Question

I'm trying to print a list of Strings all padded to the same width.

In C, I would use something like printf("%40s", cstr), where cstr is a C string.

In Swift, the best I could come up is this:

line += String(format: "%40s",string.cStringUsingEncoding(<someEncoding>))

Is there a better way ?

Answer 1

NSString has the stringByPaddingToLength: method:

line += string.stringByPaddingToLength(40, withString: " ", startingAtIndex: 0)

Answer 2

In Swift 3 you can use:

let str = "Test string"
let paddedStr = str.padding(toLength: 20, withPad: " ", startingAt: 0)

Result string: "Test string "

If you need to pad to the left the text (right justify), you can write the following function as an extension to String:

extension String {
    func leftPadding(toLength: Int, withPad character: Character) -> String {
        let newLength = self.characters.count
        if newLength < toLength {
            return String(repeatElement(character, count: toLength - newLength)) + self
        } else {
            return self.substring(from: index(self.startIndex, offsetBy: newLength - toLength))
        }
    }
}

So if you write:

let str = "Test string"
let paddedStr = str.leftPadding(toLength: 20, withPad: " ")

Result string: " Test string"

In Swift 4.1 the substring method is deprecated and there are a number of new methods to obtain a substring. Either prefix, suffix or subscripting the String with a Range<String.Index>.

For the previous extension we can use the suffix method to accomplish the same result. Since the suffix method returns a String.SubSequence, it needs to be converted into a String before being returned.

extension String {
    func leftPadding(toLength: Int, withPad character: Character) -> String {
        let stringLength = self.count
        if stringLength < toLength {
            return String(repeatElement(character, count: toLength - stringLength)) + self
        } else {
            return String(self.suffix(toLength))
        }
    }
}

Answer 3

Put all string-format-code into extension and reuse it wherever you want.

extension String {
    func padding(length: Int) -> String {
        return self.stringByPaddingToLength(length, withString: " ", startingAtIndex: 0)
    }

    func padding(length: Int, paddingString: String) -> String {
        return self.stringByPaddingToLength(length, withString: paddingString, startingAtIndex: 0)
    }
}

var str = "str"
print(str.padding(10)) // "str       "
print(str.padding(10, paddingString: "+")) // "str+++++++"

Answer 4

extension RangeReplaceableCollection where Self: StringProtocol {
    func paddingToLeft(upTo length: Int, using element: Element = " ") -> SubSequence {
        return repeatElement(element, count: Swift.max(0, length-count)) + suffix(Swift.max(count, count-length))
    }
}

---

"123".paddingToLeft(upTo: 5)              //  "  123"
"123".paddingToLeft(upTo: 5, using: "0")  //  "00123"
"123".paddingToLeft(upTo: 3, using: "0")  //    "123"
"$199.99".dropLast(3).paddingToLeft(upTo: 10, using: "_")  //  "______$199"

Answer 5

The following two functions return a string padded to the given width, either left or right justified. It is pure Swift 4, no NSString, and no C string either. You may choose if a string longer than the padding width will be truncated or not.

extension String {
    func rightJustified(width: Int, truncate: Bool = false) -> String {
        guard width > count else {
            return truncate ? String(suffix(width)) : self
        }
        return String(repeating: " ", count: width - count) + self
    }

    func leftJustified(width: Int, truncate: Bool = false) -> String {
        guard width > count else {
            return truncate ? String(prefix(width)) : self
        }
        return self + String(repeating: " ", count: width - count)
    }
}